1. Field of the Invention
This invention generally relates to integrated circuit packaging and, more particularly, to a system and method for precisely aligning a stack of tightly tolerance surface mount devices such as optical components.
2. Description of the Related Art
As noted in Wikipedia, common commercial circuit packaging includes the dual in-line package (DIP), pin, grid array (PGA), ball grid array (BGA), leadless chip carrier (LCC) packages, and surface mount package with leads formed as either a gull-wing or J-lead. A surface mount package typically occupies an area about 30-50% less than an equivalent DIP, with a typical thickness that is 70% less. This package has “gull wing” leads protruding from the two long sides and a lead spacing of 0.050 inches. In a Flip-chip Ball Grid Array (FCBGA) package the die is mounted upside-down (flipped) and connects to the package balls via a package substrate that is similar to a printed-circuit board rather than by wires. When multiple dies are combined on a small substrate, it's called an MCM, or Multi-Chip Module. A big MCM may be considered to be a small printed circuit board (PCB).
Individual components or surface mount devices (SMDs) may be placed on a die or printed circuit board (PCB) using a SMT (surface mount technology) component placement system, commonly called a pick-and-place machine. Generally, these robotic machines are used to place surface-mount devices (SMDs) onto a PCB. They are used for high speed, relatively high precision placing of broad range of electronic components, like capacitors, resistors, integrated circuits onto PCBs. These systems normally use pneumatic suction nozzles, attached to a plotter-like device to allow the nozzle head to be accurately manipulated in three dimensions. Additionally, each nozzle can be rotated independently.
Surface mount components are placed along the front (and often back) faces of the machine. Most components are supplied on paper or plastic tape, the tape reels are loaded onto feeders mounted to the machine. Through the middle of the machine there is a conveyor belt, along which blank PCBs travel. The PCB is clamped, and the nozzles pick up individual components from the feeders/trays, rotate them to the correct orientation and then place them on the appropriate pads on the PCB.
As the part is carried from the part feeders on either side of the conveyor belt to the PCB, it is photographed from below. Its silhouette is inspected and the inevitable registration errors in pickup are measured and compensated for when the part is placed. For example, if the part was shifted 0.25 mm and rotated 10° when picked up, the pickup head will adjust the placement position to place the part in the correct location.
A separate camera on the pick-and-place head photographs fiducial marks on the PCB to measure its position on the conveyor belt accurately. Two fiducial marks, measured in two dimensions each, permit the PCB's orientation and thermal expansion to be measured and compensated for as well. The components may be temporarily adhered to the PCB using the wet solder paste itself, or by using small blobs of a separate adhesive applied by a glue dispensing machine.
Some conventional tolerances are as follows. For adhesive and solder process equipment, the dimensions of the components and their tolerances are typically +/−0.1-0.2 mm. Dimension tolerances for a PCB are 0.05-0.15 mm. Conductor line widths and tolerances are 0.1-0.2 mm. High throughput pick-and-place equipment typically provides placement tolerances of 0.05-0.2 mm. Highly accurate pick-and-place equipment can provide tolerances down to 0.001 mm, but the equipment purchase price, maintenance requirements, and low throughput make their use prohibitively expensive for most commercial applications.
Unfortunately, high throughput pick-and-place machines do not currently have the placement accuracy to locate parts that must be stacked on top of each other with tight tolerances, such as optical system components. The bottom component is placed within the bounds of the inherent planar tolerance error (X) and rotational tolerance error (R) of the placement system with respect to a fiducial. An overlying component is placed within the inherent planar tolerance error (X) and rotational tolerance error (R) with respect to the bottom part, meaning that the tolerance error of the overlying part may be 2X from the intended position with a rotational misalignment as great as 2R. The placement challenges increase with the number of components on the substrate due to systemic offsets or overall statistical variability resulting in lower product manufacturing yields. Misalignment errors are further exacerbated when one or more parts on the same substrate are comprised of a fixed array of elements. The rotational tolerances translate into planar offsets that increase with the components distance to the rotational origin.
To reduce the problems arising from relying solely upon machine placement accuracies, the tight tolerance placement of parts incurs significant additional manufacturing costs since it typically must be performed on either highly accurate placement machines or on equipment that comprises active feedback loops such as computer automated vision systems or optical alignment testing. For example, a first optical component (e.g., a laser diode) must be positioned with respect to a communicating second optical component (e.g., a lens). Alignment between the components is tested by creating an optical signal. For example, a single chip or PCB is clamped into a test fixture. Upon energizing, a lid may be placed over the test fixture with an optical interface connected to a device that may, for example, measure light intensity. The relationship between components is adjusted until the test results are satisfactory, and the components can be fixed into permanent positions on the PCB. This process is often referred to as active alignment, as opposed to passive alignment where the components need not be energized or electrically tested in place. This combination of testing, aligning, and permanently fixing components is both slow and costly.
It would be advantageous if tight tolerance component stacks could be fabricated using robotic procedures, without the requirement of active alignment.